Tantalum powder is primarily used for preparing tantalum capacitors, and according to requirements in the miniaturization of electronic devices and electronic circuits and for dealing with competitions of multilayer ceramic capacitors (MLCC) and aluminum capacitors to tantalum capacitors in conventional applications, there is a need for a tantalum powder having a higher specific capacitance, a better pressure resistance and a better sintering resistance on the market. Commercial tantalum powder has a specific capacitance ranging from 8,000 to 200,000 μFV/g and a maximum consumption is directed to the tantalum powder of from 30,000 to 100,000 μFV/g, and tantalum powder having the specific capacitance of 120,000 to 200,000 μFV/g is still consumed in a less amount. There are some documents to report tantalum powder having a specific capacitance of greater than 200,000 μFV/g, while it is not applied commercially. In order to increase the pressure resistance and sintering resistance of a tantalum powder, general operations include increasing sintering temperature, prolonging sintering time, and increasing energization voltage, while increasing sintering temperature and prolong sintering time will result in the loss in the specific capacitance, and increasing the energization voltage will increase the residual current.
A commonly-used method for producing tantalum powder is to reduce potassium fluorotantalate (K2TaF7) with sodium. Particle size or specific surface area of tantalum powder is controlled by adding a diluent salt, e.g., KCl, NaCl, and KF. When the proportion of the diluent salt is increased, resultant tantalum powder will become fine, that is, to increase the surface area of the resultant tantalum powder. However, the production ability of tantalum powder will be accordingly lowered with the increase in the proportion of the diluent salt. It is economic to prepare capacitor powder having a specific capacitance of from 18,000 to 70,000 μFV/g by using sodium reduction of potassium fluorotantalate (K2TaF7) in industry. If capacitors having a high specific capacitance is required to be prepared, tantalum powder having a lower primary fineness is in need, and thus potassium fluorotantalate (K2TaF7) should be reduced with sodium at the condition of diluent salts (e.g., KCl, NaCl, and KF) in a higher proportion.
Currently, tantalum powder is primarily produced by sodium reduction of potassium fluorotantalate and magnesium reduction of tantalum oxide. The sodium reduction of potassium fluorotantalate is a conventional production process for tantalum powder, and the process has the advantages of a well-developed process and a high market share; the magnesium reduction of tantalum oxide is a novel production process, and tantalum powder produced by the method shares a certain proportion on the market. Some other production processes for tantalum powder are also reported, for example, a process of reacting TaCl5 with alkali metals, and alkali earth metals, and a process for reducing tantalum oxide with rare earth metals and/or hydrides of rare metals. However, tantalum powder produced by these processes are not commercially available.
Chinses patent ZL98802473.X (also published as U.S. Pat. No. 6,193,779) discloses a tantalum powder free of alkali and fluorine having a primary particle size of 50 to 300 nm, and a secondary particle size of more than 10 μm according to D-50 value (ASTM-B-288), which is sintered at 1100 to 1300° C. for 10 minutes and then energized with the voltage of 16V to give a capacitor having a specific capacitance of 120,000 to 180,000 μFV/g, and a residual current of 2 nA/μFV. Meantime, the patent discloses a process for preparing the tantalum powder by reacting TaCl5 with alkali metals, alkali earth metals in an inert atmosphere.
Chinese patent ZL98802572.8 (also published as U.S. Pat. No. 6,238,456) discloses a tantalum powder free of alkali and fluorine having a primary particle size of 150 to 300 nm, and a secondary particle size of more than 5 μm, which is sintered at 1200° C. for 10 minutes and then energized with the voltage of 16V to give a capacitor having a specific capacitance of 80,000 to 120,000 μFV/g, and a residual current of 5 nA/μFV. Meantime, the patent discloses a process for preparing the tantalum powder by reducing potassium fluorotantalate with metal sodium.
Japanese patent JP4828016 (also published as PCT/JP01/06768, WO 02/11932) discloses a process of producing tantalum powder by using metal sodium to reduce potassium fluorotantalate, and the tantalum powder prepared by the patent process has a specific capacitance of 80,000 to 250,000 μFV/g.
International patent WO 2010/148627A1(PCT/CN2010/000414) discloses a method of preparing tantalum powder for a high capacitance capacitor with three-step reduction, wherein tantalum oxide is reduced with rare earth metals and/or hydrides of rare earth metals, and the method can prepare tantalum powder having a specific capacitance of 100,000 to 400,000 μFV/g.
The above patents have the following disadvantages, and thus the corresponding processes therein have certain limits during actual applications.
Tantalum powder as manufactured in Chinese patent ZL98802473.X (also published as U.S. Pat. No. 6,193,779) has low particle size, poor sintering resistance and low energization voltage.
Tantalum powder as manufactured in Chinese patent ZL98802572.8 (also published as U.S. Pat. No. 6,238,456) has a low specific capacitance, and a large residual current.
In the method of Japanese patent JP4828016 (also published as PCT/JP01/06768, WO02/11932), prior to the addition of sodium, the amount of diluent salts is greater than the amount of potassium fluorotantalate (K2TaF7) by a factor of 40 to 1000, and thus the method is not economical. In addition, the patent only discloses the specific capacitance of the tantalum powder prepared in the process, while it does not disclose the residual current of the tantalum powder.
The process for producing tantalum powder in International patent WO 2010/148627A1 (PCT/CN2010/000414) has a high requirement to the quality of the raw material tantalum powder, and thus the performances of the resultant tantalum powder will depend on tantalum oxide. Furthermore, the process is more complex than the sodium reduction of potassium fluorotantalate.